17 1 Conic Sections

ContentsContents 1717

Polar Coordinates Conics and

17.1 Conic Sections 2

17.2 Polar Coordinates 23

17.3 Parametric Curves 33

Learning

In this Workbook you will learn about some of the most important curves in the whole ofmathematics - the conic sections: the ellipse, the parabola and the hyperbola. You willlearn how to recognise these curves and how to describe them in Cartesian and in polarform. In the final block you will learn how to describe cruves using a parametric approachand, in particular, how the conic sections are described in parametric form.

outcomes

Conic Sections

17.1

IntroductionThe conic sections (or conics) - the ellipse, the parabola and the hyperbola - play an importantrole both in mathematics and in the application of mathematics to engineering. In this Section welook in detail at the equations of the conics in both standard form and general form.

Although there are various ways that can be used to define a conic, we concentrate in this Section ondefining conics using Cartesian coordinates (x, y). However, at the end of this Section we examinean alternative way to obtain the conics.

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&

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%

PrerequisitesBefore starting this Section you should . . .

be able to factorise simple algebraicexpressions

be able to change the subject in simplealgebraic equations

be able to complete the square in quadraticexpressions'

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%

Learning OutcomesOn completion you should be able to . . .

understand how conics are obtained as curvesof intersection of a double-cone with a plane

state the standard form of the equations ofthe ellipse, the parabola and the hyperbola

classify quadratic expressions in x, y in termsof conics

2 HELM (2008):Workbook 17: Conics and Polar Coordinates

1. The ellipse, parabola and hyperbolaMathematicians, engineers and scientists encounter numerous functions in their work: polynomials,trigonometric and hyperbolic functions amongst them. However, throughout the history of sci-ence one group of functions, the conics, arise time and time again not only in the development ofmathematical theory but also in practical applications. The conics were first studied by the Greekmathematician Apollonius more than 200 years BC.

Essentially, the conics form that class of curves which are obtained when a double cone is intersectedby a plane. There are three main types: the ellipse, the parabola and the hyperbola. From theellipse we obtain the circle as a special case, and from the hyperbola we obtain the rectangularhyperbola as a special case. These curves are illustrated in the Figures 1 and 2.

Circle: obtained by intersection of a planeperpendicular to the cone-axis with cone.

As the plane of intersection tilts the otherconics are obtained:

Ellipse: obtained by a plane, which is notperpendicular to the cone-axis, but cuttingthe cone in a closed curve.Various ellipses are obtained as the planecontinues to rotate.

cone-axis

generator lines

plane of intersection

(A degenerate case is a single point.)

Figure 1: Circle and ellipse

HELM (2008):Section 17.1: Conic Sections

3

Parabola: obtained when the plane isparallel to the generator of the cone.Different parabolas are obtained as the pointP moves along a generator.

cone-axis

generator line

P

cone-axis

generator line

Hyperbola: obtained when the planeintersects both parts of the cone. Therectangular hyperbola is obtained when theplane is parallel to the cone-axis.(A degenerate case is two straight lines.)

Figure 2: Parabola and hyperbola

The ellipseWe are all aware that the paths followed by the planets around the sun are elliptical. However, moregenerally the ellipse occurs in many areas of engineering. The standard form of an ellipse is shownin Figure 3.

ae a

ae aea a

ex

y

b

b

directrixfoci

minor-axis

major-axis

directrix

Figure 3


If a > b (as in Figure 1) then the x-axis is called the major-axis and the y-axis is called the minor-axis. On the other hand if b > a then the y-axis is called the major-axis and the x-axis is thenthe minor-axis. Two points, inside the ellipse are of importance; these are the foci. If a > b theseare located at coordinate positions ae (or at be if b > a) on the major-axis, with e, called theeccentricity, given by

e2 = 1 b2

a2(b < a) or by e2 = 1 a

2

b2(a < b)

The foci of an ellipse have the property that if light rays are emitted from one focus then on reflectionat the elliptic curve they pass through at the other focus.

Key Point 1

The standard Cartesian equation of the ellipse with its centre at the origin isx2

a2+y2

b2= 1

This ellipse has intercepts on the x-axis at x = a and on the y-axis at b. The curve is alsosymmetrical about both axes. The curve reduces to a circle in the special case in which a = b.

Example 1(a) Sketch the ellipse

x2

4+y2

9= 1 (b) Find the eccentricity e

(c) Locate the positions of the foci.

Solution

(a) We can calculate the values of y as x changes from 0 to 2:

x 0 0.30 0.60 0.90 1.20 1.50 1.80 2

y 3 2.97 2.86 2.68 2.40 1.98 1.31 0

From this table of values, and using the symmetry of the curve, a sketch can be drawn (see Figure4). Here b = 3 and a = 2 so the y-axis is the major axis and the x-axis is the minor axis.

Here b = 3 and a = 2 so the y-axis is the major axis and the x-axis is the minor axis.

(b) e2 = 1 a2/b2 = 1 4/9 = 5/9 e =

5/3

(c) Since b > a and be =

5, the foci are located at

5 on the y-axis.


5

Solution (contd.)

foci

5

5

2 2

3

3

x

y

Figure 4

Key Point 1 gives the equation of the ellipse with its centre at the origin. If the centre of the ellipsehas coordinates (, ) and still has its axes parallel to the x- and y-axes the standard equationbecomes

(x )2

a2+

(y )2

b2= 1.

Task

Consider the points A and B with Cartesian coordinates (c, 0) and (c, 0) re-spectively. A curve has the property that for every point P on it the sum of thedistances PA and PB is a constant (which we will call 2a). Derive the Cartesianform of the equation of the curve and show that it is an ellipse.

AB c

P (x, y)

cO

x

y

Your solution


AnswerWe use Pythagorass theorem to work out the distances PA and PB:

Let R1 = PB = [(x + c)2 + y2]1/2 and let R2 = PA = [(c x)2 + y2]1/2

We now take the given equation R1 + R2 = 2a and multiply both sides by R1 R2. The quantityR21R22 on the left is calculated to be 4cx, and 2a(R1R2) is on the right. We thus obtain a pairof equations: R1 +R2 = 2a and R1 R2 =

2cx

a

Adding these equations together gives R1 = a+cx

aand squaring this equation gives

x2 + c2 + 2cx + y2 = a2 +c2x2

a2+ 2cx

Simplifying: x2(1 c2

a2) + y2 = a2 c2 whence x

2

a2+

y2

(a2 c2)= 1

This is the standard equation of an ellipse if we set b2 = a2 c2, which is the traditional equationwhich relates the two semi-axis lengths a and b to the distance c of the foci from the centre of theellipse.

The foci A and B have optical properties; a beam of light travelling from A along AP and undergoinga mirror reflection from the ellipse at P will return along the path PB to the other focus B.

The circleThe circle is a special case of the ellipse; it occurs when a = b = r so the equation becomes

x2

r2+y2

r2= 1 or, more commonly x2 + y2 = r2

Here, the centre of the circle is located at the origin (0, 0) and the radius of the circle is r. If thecentre of the circle at a point (, ) then the equation takes the form:

(x )2 + (y )2 = r2

Key Point 2

The equation of a circle with centre at (, ) and radius r is (x )2 + (y )2 = r2


7

Task

Write down the equations of the five circles (A to E) below:

circle A

circle B

circle C circle D

circle E

2

1

1

2.5

0.5 1 2 3 x

y

1

1 1

10.5

0.52

Your solution

AnswerA (x 1)2 + (y 1)2 = 1

B (x 3)2 + (y 1)2 = 1

C (x + 0.5)2 + (y + 2)2 = 1

D (x 2)2 + (y + 2)2 = (0.5)2

E (x + 0.5)2 + (y 2.5)2 = 1


Example 2Show that the expression

x2 + y2 2x + 6y + 6 = 0

represents the equation of a circle. Find its centre and radius.

Solution

We shall see later how to recognise this as the equation of a circle simply by examination of thecoefficients of the quadratic terms x2, y2 and xy. However, in the present example we will use theprocess of completing the square, for x and for y, to show that the expression can be written instandard form.

Now x2 + y2 2x + 6y + 6 x2 2x + y2 + 6y + 6.

Also,

x2 2x (x 1)2 1 and y2 + 6y (y + 3)2 9.

Hence we can write

x2 + y2 2x + 6y + 6 (x 1)2 1 + (y + 3)2 9 + 6 = 0

or, taking the free constants to the right-hand side:

(x 1)2 + (y + 3)2 = 4.

By comparing this with the standard form we conclude this represents the equation of a circle withcentre at (1,3) and radius 2.

Task

Find the centre and radius of each of the following circles:

(a) x2 + y2 4x 6y = 12 (b) 2x2 + 2y2 + 4x + 1 = 0

Your solution

Answer

(a) centre: (2, 3) radius 1 (b) centre: (1, 0) radius

2/2.


9

Engineering Example 1

A circle-cutting machine

Introduction

A cutting machine creates circular holes in a piece of sheet-metal by starting at the centre of thecircle and cutting its way outwards until a hole of the correct radius exists. However, prior to cutting,the circle is characterised by three points on its circumference, rather than by its centre and radius.Therefore, it is necessary to be able to find the centre and radius of a circle given three points thatit passes through.

Problem in words

Given three points on the circumference of a circle, find its centre and radius

(a) for three general points

(b) (i) for (6, 5), (3, 6) and (2, 1) (ii) for (0.7, 0.6), (5.9, 1.4) and (0.8,2.8)

where coordinates are in cm.

Mathematical statement of problem

A circle passes through the three points. Find the centre (x0, y0) and radius R of this circle whenthe three circumferential points are

(a) (x1, y1), (x2, y2) and (x3, y3)

(b) (i) (6, 5), (3, 6) and (2, 1)(ii) (0.7, 0.6), (5.9, 1.4) and (0.8,2.8)Measurements are in centimetres; give answers correct to 2 decimal places.

Mathematical analysis

(a) The equation of a circle with centre at (x0, y0) and radius R is

(x x0)2 + (y y0)2 = R2

and, if this passes through the 3 points (x1, y1), (x2, y2) and (x3, y3) then

(x1 x0)2 + (y1 y0)2 = R2 (1)(x2 x0)2 + (y2 y0)2 = R2 (2)(x3 x0)2 + (y3 y0)2 = R2 (3)

Eliminating the R2 term between (1) and (2) gives

(x1 x0)2 + (y1 y0)2 = (x2 x0)2 + (y2 y0)2

so that

x21 2x0x1 + y21 2y0y1 = x22 2x0x2 + y22 2y0y2 (4)


Similarly, eliminating R2 between (1) and (3) gives

x21 2x0x1 + y21 2y0y1 = x23 2x0x3 + y23 2y0y3 (5)

Re-arranging (4) and (5) gives a system of two equations in x0 and y0.

2(x2 x1)x0 + 2(y2 y1)y0 = x22 + y22 x21 y21 (6)

2(x3 x1)x0 + 2(y3 y1)y0 = x23 + y23 x21 y21 (7)

Multiplying (6) by (y3 y1), and multiplying (7) by (y2 y1), subtracting and re-arranging gives

x0 =1

2

((y3 y1)(x22 + y22) + (y1 y2)(x23 + y23) + (y2 y3)(x21 + y21)

x2y3 x3y2 + x3y1 x1y3 + x1y2 x2y1

)(8)

while a similar procedure gives

y0 =1

2

((x1 x3)(x22 + y22) + (x2 x1)(x23 + y23) + (x3 x2)(x21 + y21)

x2y3 x3y2 + x3y1 x1y3 + x1y2 x2y1

)(9)

Knowing x0 and y0, the radius R can be found from

R =

(x1 x0)2 + (y1 y0)2 (10)

(or alternatively using x2 and y2 (or x3 and y3) as appropriate).

Equations (8), (9) and (10) can now be used to analyse the two particular circles above.

(i) Here x1 = 6 cm, y1 = 5 cm, x2 = 3 cm, y2 = 6 cm, x3 = 2 cm and y3 = 1 cm, so that

x2y3 x3y2 + x3y1 x1y3 + x1y2 x2y1 = 3 12 + 10 + 6 36 + 15 = 20

and

x21 + y21 = 61 x

22 + y

22 = 45 x

23 + y

23 = 5

From (8)

x0 =1

2

(4 45 + (1) 5 + 5 61

20

)=180 5 + 305

40= 3

while (9) gives

y0 =1

2

(8 45 + 3 5 + 5 61

20

)=360 + 15 + 305

40= 1

The radius can be found from (10)

R =

(6 (3))2 + (5 1)2 =

25 = 5

so that the circle has centre at (3, 1) and a radius of 5 cm.


11

(ii) Now x1 = 0.7 cm, y1 = 0.6 cm, x2 = 5.9 cm, y2 = 1.4 cm, x3 = 0.8 cm and y3 = 2.8 cm,so that

x2y3x3y2+x3y1x1y3+x1y2x2y1 = 16.521.12+0.481.960.983.54 = 23.64

and

x21 + y21 = 0.85 x

22 + y

22 = 36.77 x

23 + y

23 = 8.48

so from (8)

x0 =1

2

(125.018 6.784 + 3.57

23.64

)=128.23247.28

= 2.7121827

and from (9)

y0 =1

2

(55.155 + 55.968 4.335

23.64

)=3.52247.28

= 0.0744924

and from (10)

R =

(0.7 2.7121827)2 + (0.6 0.0744924)2 =

11.9191490 = 3.4524121

so that, to 2 d.p., the circle has centre at (2.71, 0.07) and a radius of 3.45 cm.

Mathematical comment

Note that the expression

x2y3 x3y2 + x3y1 x1y3 + x1y2 x2y1appears in the denominator for both x0 and y0. If this expression is equal to zero, the calculationwill break down. Geometrically, this corresponds to the three points being in a straight line so thatno circle can be drawn, or not all points being distinct so no unique circle is defined.


Engineering Example 2

The web-flange junction

Introduction

In problems of torsion, the torsion constant, J , which is a function of the shape and structure ofthe element under consideration, is an important quantity.A common beam section is the thick I-section shown here, forwhich the torsion constant is given by

J = 2J1 + J2 + 2D4

where the J1 and J2 terms refer to the flanges and web respec-tively, and the D4 term refers to the web-flange junction. Infact

= min

[tftw,twtf

](0.15 + 0.1

r

tf

)

flange

flange

web

where tf and tw are the thicknesses of the flange and web respectively, and r is the radius of theconcave circle element between the flange and the web. D is the diameter of the circle of theweb-flange junction.

pp

pppppp pp

pp

pp

pppp

pppppp

- D

QQk qr -tw

?

6tf

As D occurs in the form D4, the torsion constant is very sensitive to it. Calculation of D is thereforea crucial part of the calculation of J .

Problem in words

FindD, the diameter of the circle within the webflange junction as a function of the other dimensionsof the structural element.

Mathematical statement of problem

(a) Find D, the diameter of the circle, in terms of tf and tw (the thicknesses of the flange and theweb respectively) in the case where r = 0. When tf = 3cm and tw = 2cm, find D.

(b) For r 6= 0, find D in terms of tf , tw and r. In the special case where tf = 3 cm, tw = 2 cmand r = 0.4 cm, find D.


13

Mathematical analysis

(a) Consider a co-ordinate system based on the midpoint of the outer surface of the flange.

ppp p pp pppp pp pp pppppppppp -R6

y

- x

sA

The centre of the circle will lie at (0,R) where R is the radius of the circle, i.e. R = D/2.The equation of the circle is

x2 + (y +R)2 = R2 (1)

In addition, the circle passes through the corner at point A (tw/2,tf ), so(tw2

)2+ (tf +R)2 = R2 (2)

On expanding

t2w4

+ t2f 2Rtf +R2 = R2

giving

2Rtf =t2w4

+ t2f R =(t2w/4) + t

2f

2tf=

t2w8tf

+tf2

so that

D = 2R =t2w4tf

+ tf (3)

Setting tf = 3 cm, tw = 2 cm gives

D =22

4 3+ 3 = 3.33 cm


(b) Again using a co-ordinate system based on the mid-point of the outer surface of the flange,consider now the case r 6= 0.

ppp ppppp pppppp

pppp

pppppp6

y

- x

rB-R

r

Point B (tw/2 + r,tf r) lies, not on the circle described by (1), but on the slightly largercircle with the same centre, and radius R + r. The equation of this circle is

x2 + (y +R)2 = (R + r)2 (4)

Putting the co-ordinates of point B into equation (4) gives(tw2

+ r

)2+ (tf r +R)2 = (R + r)2 (5)

which, on expanding gives

t2w4

+ twr + r2 + t2f + r

2 +R2 + 2tfr 2tfR 2rR = R2 + 2rR + r2

Cancelling and gathering terms gives

t2w4

+ twr + r2 + t2f + 2tfr = 4rR + 2tfR

= 2R (2r + tf )

so that

2R = D =(t2w/4) + twr + r

2 + t2f + 2tfr

(2r + tf )

so D =t2w + 4twr + 4r

2 + 4t2f + 8tfr

(8r + 4tf )(6)

Now putting tf = 3 cm, tw = 2 cm and r = 0.4 cm makes

D =22 + (4 2 0.4) + (4 0.42) + (4 32) + (8 3 0.4)

(8 0.4) + (4 3)=

53.44

15.2= 3.52 cm

Interpretation

Note that setting r = 0 in Equation (6) recovers the special case of r = 0 given by equation (3).The value of D is now available to be used in calculations of the torsion constant, J .


15

The parabolaThe standard form of the parabola is shown in Figure 5. Here the x-axis is the line of symmetry ofthe parabola.

focus

a a x

y

directrix

Figure 5

Key Point 3

The standard equation of the parabola with focus at (a, 0) is

y2 = 4ax

It can be shown that light rays parallel to the x-axis will, on reflection from the parabolic curve, cometogether at the focus. This is an important property and is used in the construction of some kindsof telescopes, satellite dishes and car headlights.

Task

Sketch the curve y2 = 8x. Find the position of the focus and confirm its light-focusing property.

Your solution


AnswerThis is a standard parabola (y2 = 4ax) with a = 2. Thus the focus is located at coordinate position(2, 0).

focus2 x

y

If your sketch is sufficiently accurate you should find that light-rays (lines) parallel to the x-axiswhen reflected off the parabolic surface pass through the focus. (Draw a tangent at the point ofreflection and ensure that the angle of incidence ( say) is the same as the angle of reflection.)

By changing the equation of the parabola slightly we can change the position of the parabola alongthe x-axis. See Figure 6.

y2 = 4a(x + 1) y2 = 4axy2 = 4a(x 3)

1 3 x

y

Figure 6: Parabola y = 4a(x b) with vertex at x = b

We can also have parabolas where the y-axis is the line of symmetry (see Figure 7). In this case thestandard equation is

x2 = 4ay or y =x2

4a

focusa

x

y

Figure 7


17

Task

Sketch the curves y2 = x and x2 = 2(y 3).

Your solution

Answer

x

y

y2 = x

x2 = 2(y 3)

3

The focus of the parabola y2 = 4a(x b) is located at coordinate position (a+ b, 0). Changing thevalue of a changes the convexity of the parabola (see Figure 8).

x

y

y2 = x

y2 = 2x

y2 = 3x

Figure 8


The hyperbolaThe standard form of the hyperbola is shown in Figure 9(a).

This has standard equation

x2

a2 y

2

b2= 1

The eccentricity, e, is defined by

e2 = 1 +b2

a2(e > 1)

focus

ae a a ae x

y

focus focus

x

y

focusb

b

asymptotes

(a) (b)

Figure 9

Note the change in sign compared to the equivalent expressions for the ellipse. The lines y = bax

are asymptotes to the hyperbola (these are the lines to which each branch of the hyperbola approachas x ).

If light is emitted from one focus then on hitting the hyperbolic curve it is reflected in such a wayas to appear to be coming from the other focus. See Figure 9(b). The hyperbola has fewer uses inapplications than the other conic sections and so we will not dwell here on its properties.

Key Point 4

The standard equation of the hyperbola with foci at (ae, 0) is

x2

a2 y

2

b2= 1 with eccentricity e given by e2 = 1 +

b2

a2(e > 1)


19

General conicsThe conics we have considered above - the ellipse, the parabola and the hyperbola - have all beenpresented in standard form:- their axes are parallel to either the x- or y-axis. However, conics maybe rotated to any angle with respect to the axes: they clearly remain conics, but what equations dothey have?It can be shown that the equation of any conic, can be described by the quadratic expression

Ax2 +Bxy + Cy2 +Dx + Ey + F = 0

where A,B,C,D,E, F are constants.

If not all of A,B,C are zero (and F is a suitable number) the graph of this equation is

(i) an ellipse if B2 < 4AC (circle if A = C and B = 0)

(ii) a parabola if B2 = 4AC

(iii) a hyperbola if B2 > 4AC

Example 3Classify each of the following equations as ellipse, parabola or hyperbola:

(a) x2 + 2xy + 3y2 + x 1 = 0(b) x2 + 2xy + y2 3y + 7 = 0(c) 2x2 + xy + 2y2 2x + 3y = 6(d) 3x2 + 2x 5y + 3y2 10 = 0

Solution

(a) Here A = 1, B = 2, C = 3 B2 < 4AC. This is an ellipse.

(b) Here A = 1, B = 2, C = 1 B2 = 4AC. This is a parabola.

(c) Here A = 2, B = 1, C = 2 B2 < 4AC also A = C but B 6= 0. This is an ellipse.

(d) Here A = 3, B = 0, C = 3 B2 < 4AC. Also A = C and B = 0. This is a circle.


Task

Classify each of the following conics:

(a) x2 2xy 3y2 + x 1 = 0

(b) 2x2 + xy y2 2x + 3y = 0

(c) 4x2 y + 3 = 0

(d) x2 xy y2 + 3x = 0

(e) 2x2 + 2y2 x + 3y = 7

Your solution

Answer

(a) A = 1, B = 2, C = 3 B2 > 4AC hyperbola

(b) A = 2, B = 1, C = 1 B2 > 4AC hyperbola

(c) A = 4, B = 0, C = 0 B2 = 4AC parabola

(d) A = 1, B = 1, C = 1 B2 < 4AC, A = C, B 6= 0 ellipse

(e) A = 2, B = 0, C = 2 B2 < 4AC, A = C and B = 0 circle


21

Exercises

1. The equation 9x2 + 4y2 36x + 24y 1 = 0 represents an ellipse. Find its centre, thesemi-major and semi-minor axes and the coordinate positions of the foci.

2. Find the equation of a circle of radius 3 which has its centre at (1, 2.2)

3. Find the centre and radius of the circle x2 + y2 2x 2y 5 = 0

4. Find the position of the focus of the parabola y2 x + 3 = 0

5. Classify each of the following conics

(a) x2 + 2x y 3 = 0(b) 8x2 + 12xy + 17y2 20 = 0(c) x2 + xy 1 = 0(d) 4x2 y2 4y = 0(e) 6x2 + 9y2 24x 54y + 51 = 0

6. An asteroid has an elliptical orbit around the Sun. The major axis is of length 5 108 km. Ifthe distance between the foci is 4 108 km find the equation of the orbit.

Answers

1. centre: (2,3), semi-major 3, semi-minor 2, foci: (2,3

5)

2. (x + 1)2 + (y 2.2)2 = 9

3. centre: (1, 1) radius

7

4. y2 = (x 3) a = 1, b = 3. Hence focus is at coordinate position (4, 0).

5. (a) parabola with vertex (1,4)(b) ellipse

(c) hyperbola

(d) hyperbola

(e) ellipse with centre (2, 3)

6. 9x2 + 25y2 = 5.625 107


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17 1 Conic Sections

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